We need some air in here!

In a previous posting I mentioned running into an issue with our water
heater failing to draft when all the exhaust fans and clothes dryer
running. I did some research and through testing I thought the entire
issue could be resolved by adding more “make-up air”. Thus I added a
6″ duct into our mechanical room. This was in addition to our existing
5″. You would think that after having ~48″ sq. of open hole to the
outside would mitigate any possible negative pressure, but you would
be wrong. The water heater still failed to draft in worst case CAZ
testing. I called a local HVAC contractor to evaluate. They spent
3 hours trying different things, including adding 3′ to our chimney,
but no joy!